Semiconductor chips such as ICs and LSIs are usually as small as several millimeters square and about 100 microns thick and are difficult to mount as they are on printed-circuit boards. To facilitate the mounting, it is customary to contain each chip in a certain housing known as an IC or LSI package.
The IC or LSI package basically has a structure in which a semiconductor chip is attached to a heat sink, or a heat-radiating metal sheet, and electrode terminals of the chip and leads for connection to external circuits are joined by bonding wires.
The leads project from the package like the legs of a centipede, and are also called pins.
Such IC and LSI packages prevailing at present day are classified into two dominant types; dual in-line package (DIP) having two rows of pins extending vertically downward from the package body at opposite sides thereof and flat package (FP) with pins projecting from all four edges of the package body in the same plane.
The FP type is advantageous over the DIP type because it can use some more leads (pins) and thereby slightly increase the packaging density on printed boards.
However, the recent tendency toward even higher degrees of integration for LSIs has accordingly increased the number of pins for the individual packages. The FP and DIP types are both failing to keep up with the tendency, and there is a need for a new packaging system capable of accommodating the rapidly increasing number of pins.
In an effort to meet the requirement, a new mounting and packaging system using a "film carrier" (also known as tape carrier or tab) has been developed.
The film carrier, as shown in FIG. 1, is based on a length of tape 2 formed with sprocket holes 1. The tape 2, or the base using a polyimide, polyester, polyether sulfone (PES), polyparabanic acid (PPA) or such like resin, is covered with a copper foil. The foil in turn is photoetched to provide copper inner leads (fingers for chip bonding) 3 and copper outer leads (fingers for external connections) 4. The numeral 5 indicates testing pads.
The inner leads 3, outer leads 4, and the like in a fine pattern are collectively called leads in this specification.
The process steps in common use will now be described in some more detail. The Polyimide or other resin base of a lengthy tape is perforated to provide device holes, a copper foil about 35 micron thick is laminated as a circuit-forming metal to the perforated tape, and the copper foil is coated with a resist, a pattern is printed, exposed to light, developed, and etched. After the removal of the resist and, where necessary, after an additional step of plating, a fine pattern of leads as in FIG. 1 is formed.
As FIG. 1 illustrates, a fragment of the base film is punched in the center to provide a device hole for mounting a semiconductor chip or the like, and leads formed from the copper foil are arranged in a high density in the hole to partly project thereinto. The width of the leads is sometimes as narrow as several ten microns.
At the electrodes of a semiconductor chip there are usually formed bumps for connection to the inner leads on the film carrier. The electrodes (bumps) of the semiconductor chip and the inner leads of the film carrier are joined by the gang bonding method for simultaneous connection of all terminals involved. When the leads are to be mounted on a printed-circuit board, the outer leads of copper foil are cut out together with the semiconductor element from the film carrier (by punching) and then mounted on the printed board.
The tape carrier formed this way offers many advantages including the following:
(1) It can be handled in the form of a (long) tape and precisely positioned as desired using the sprocket holes.
(2) Unlike wire bonding, the bonding seldom deforms the inner leads and permits the provision of terminals to a much finer pitch (on the order of 80 microns).
(3) The gang bonding allows for a single-step bonding regardless of the number of terminals involved.
(4) Burn-in tests of the chips as attached to the carrier are possible.
(5) The thin, flexible carrier permits correspondingly thin, flexible type of packages.
(6) The chips after packaging can be easily replaced.
This film carrier is particularly well suited for high-density package type LSIs that require larger numbers of pins than usual.
For the metal conductors (leads) to be used on the above film carrier, which are required to have high electrical conductivity, tough pitch copper foils 20 to 50 .mu.m thick have hitherto been employed. However, the tough pitch copper (pure copper) foils have number of drawbacks and none have proved satisfactory. In order to attain a high density arrangement of fine leads it is necessary to secure enhanced etching accuracy. To this end the copper foil must be made as thin as possible. Nevertheless, the fine copper leads, formed by photoetching the copper foil about 20 to 50 .mu.m thick as stated above, tend to soften on heating during the course of fabrication. Also they are easily deformed as the resist is peeled off during etching of when the flow of plating solution changes or the film carrier comes in contact with rolls conveying it. The deformation of the fingers can lead to shorting of the terminals or imperfect bonding.
When a copper foil is bonded to a resin base with an adhesive, intimate adhesion between the foil and resin is obtained and reliability improved by the use of a high curing temperature and an adhesive for high temperature use. However, this involves a relatively long curing period (usually several hours) and presents a problem of easy softening of the ordinary pure copper foils.
An additional disadvantage with the conventional rolled copper foils forming the leads has been anisotropy in mechanical properties. There are sharp distinctions between their longitudinal (rolling direction) and lateral (normal to the rolling direction) tensile strengths and elongations.
In view of the foregoing and other considerations, the copper foil as a metal conductor on a tape carrier is required to possess the following properties:
(1) High electrical conductivity as a metal conductor.
(2) Toward the requirement for thinner foils, greater strength than pure copper, and no possibility of deformation during fabrication.
(3) Sufficient heat resistance to withstand the heat of approximately 200.degree. C. to be encountered during the manufacture of the tape carrier.
(4) No anisotropy in strength or heat resistance, which is essential for the fingers extended in four directions.
(5) Smooth surface to permit the bonding of IC elements at the back of the fingers.
(6) Flatness of the shape for the same purpose as above.
(7) Ease of etching.
(8) Good adhesion to the resin involved.
As far as conventional tough pitch copper (pure copper basis) foil concerned, it is impossible to satisfy with all requirements stated above.
In this connection, tough pitch copper foils generally used have a following compositions:
Cu: 99.96-99.97 wt % PA1 O.sub.2 : 0.025-0.035 wt % PA1 concomitant impurities: not more than 0.05 wt % (Fe, Pb, etc.)